1. Field of the Invention
The present invention relates generally to mass flow rate and density measuring apparatus, and more particularly to an improved flow rate sensor including means for substantially improving the sensitivity of the device.
2. Description of Prior Art
Numerous attempts have been made in the last twenty years or so to provide Coriolis type mass flow rate sensing apparatus in which various configurations of straight tubes, U-shaped tubes, looped tubes, etc., have been either rotated or oscillated in a controlled manner such that Coriolis induced deflection can be measured (or the effects of such deflections can be measured) as an indication of mass flow rate through the tubes. An extensive listing of prior art patents relating to Coriolis devices is given in the U.S. patent to James E. Smith, U.S. Pat. No. Re. 31,450, which issued Nov. 29, 1983.
As is well pointed out in the Smith, Cox and Sipin Patents listed in the above reference, certain asdvantages can be obtained by constructing the sensor device in a tuning fork configuration and driving the device at a frequency at or near the resonant frequency in the "drive mode" thereof. The Smith U.S. Pats. No. 4,187,721; U.S. Pat. No. Re. 31,450, and U.S. Pat. No. 4,422,338 teach tuning fork structures in which a single conduit forms one arm of the tuning fork, and a resilient spring arm forms the second element of the tuning fork. In such structures, the resilient U-shaped conduit through which the mass to be sensed flows is vibrated against the resilient arm. In other prior art patents of Smith (U.S. Pat. No. 4,252,028) and Cox (U.S. Pat. Nos. 4,127,028; 4,192,184 and 4,311,054), a second U-shaped tube is substituted for the spring arm and the two tubes are driven relative to each other. In both types of structures a magnet is affixed to one of the arms and a magnetic driving means is affixed to the other for interacting with the magnet and applying driving forces between the two tubes.
Similarly, in both types of structures, in order to sense Coriolis induced distortion between the tubes, flags or magnets are similarly affixed to points on one of the tubes and sensing detectors or coils are correspondingly affixed to the other tube such that relative motion therebetween is sensed by the EMF induced in the sensing windings. Both the drive and sensing elements obviously have mass, and in some cases such mass can have a significant impact on the dynamic response characteristics of the structure.
Furthermore, in order to communicate electrical current to the driving coil, and to obtain induced signals from the sensing windings, conductive wires are typically glued or taped directly to the tubes or other vibrating parts of the sensor and extended therealong to the tube mounting points for distribution to an electronic control and detection part of the system. This gives rise to disadvantages in addition to mass related effects in that the wires and their attachment means are subject to detachment as the tubes are vibrated during operation. Furthermore, in some cases the conductivity of the conductors may even be influenced by the temperature of the material flowing through the tubes.
It is well known that in systems in which the sensing tube is driven about one oscillating "axis" (drive mode) and the Coriolis forces induce oscillation about another "axis" (Coriolis mode), the structure will exhibit resonance characteristics in the drive mode that are different from the resonance characteristics in the Coriolis mode. One of such characteristics is that the natural frequencies of the two modes are different. If is to be understood that some tube configurations, such as the helical or quasi-helical loop devices of the type disclosed in the above identified copending applications, do not have definable straight-line axes of oscillation.
In any given tube structure the relationship between the natural frequency of the drive mode (W.sub.D) and the natural frequency of the Coriolis mode (W.sub.C) are predetermined by the particular vibrational characteristics of the structure. Such a relationship can be graphically expressed in a Coriolis mode resonance diagram of the type shown in FIG. 6 of the drawing wherein the mechanical amplification factor H is plotted against the ratio of W.sub.D to W.sub.C. As indicated, the amplification factor (proportional to the sensitivity to mass flow rate) of the device is determined by the point at which the drive frequency W.sub.D falls on the curve C. However, since the frequencies W.sub.D and W.sub.C will change somewhat with changes in density of the mass flow through the tube, it can be expected that the sensitivity of the device will also change with density. This is undesirable because it can lead to substantial inaccuracies where large swings in density are encountered.
Attempts have been made in the prior art to provide a density independent structure by tailoring the sensor structure so that it has a drive frequency that is remote from the Coriolis frequency and thus falls within the shaded portion S of the curve C (FIG. 6). The U-tube devices disclosed in the Smith U.S. Pats. Nos. 4,187,721, 4,252,028, 4,422,338, and 4,491,025 have used such technique. Although a degree of density independence is achieved using such an approach, it results in a device which has a relative low sensitivity characteristic. Consequently, devices of this type are not suitable for use with low mass materials such as gases, for example.
Other attempts to improve sensitivity have involved the coice of various configurations which are inherently more sensitive than the Smith U-tube configurations. For example, see the U.S. Pat. No. 4,127,028 to Cox et al, wherein sensitivity was increased by causing the legs of a U-tube to converge in the region near the attachment points. Although providing a degree of improvement over the U-tube approach, the Cox device was subject to other limitations and has not met with practical success.
Although the above stated disadvantages can usually be tolerated in larger conduit applications, in flow meters for measuring small flow rates and utilizing small diameter tubing, e.g., one-eighth inch, one-sixteenth inch, etc., the mass of the driving coils and sense coils, and their associated wires, etc., comprises an appreciable fraction of the conduit mass. This tends to upset and deleteriously influence the conduit's vibrational and dynamic characteristics, sometimes to the point where it is impossible to build acceptable meters with the tube diameters of the stated size.